Metal Semiconductor Field Effect Transistors (MESFETs) are included in high-frequency circuits utilized in cellular phones, pagers, and other RF devices. A MESFET is formed on the major surface of a semiconductor gallium arsenide (GaAs) substrate having overlying buffer and active layers formed by ion implantation or epitaxial deposition. Ohmic metal contacts are utilized for source and drain contacts and a gate is formed by a Schottky gate metal contact.
FIGS. 1A and 1B depict the vertical structure and band levels of the active layer and buffer layer. Referring to FIG. 1A, the active layer 16 has a top depletion region 30 of width W.sub.G at the interface with the gate metal contact 24 due to the voltage bias on the gate. There is also a bottom depletion layer region 32 at the interface of the active layer 16 and the buffer layer 14. This bottom depletion region 32 extends in the active layer 16 a distance W.sub.n and into the buffer layer 16 a distance W.sub.s.
FIG. 1B shows the band bending near the interface between the channel 16 and the buffer layer 14 or substrate 12. When a gate bias is applied on the gate metal contact 24 there is some voltage change on the channel/buffer interface. This voltage difference (the difference between EFV and EFO) quickly becomes zero but normally there is a time transient due to the deep traps at the interface or in other regions and when the buffer in not a perfect ground with the source.
The time transient of the substrate band bending might strongly influence the basic performance of the GaAs MESFET especially RF performance characteristics such as transconductance reduction, anomalies of backgating and hysterisis of the drain current.
Reduction of the band banding at the channel/substrate interface also would improve device linearity, reduce drain current hysterisis, and provide higher gain near the MESFET pinchoff voltage.
Some techniques to reduce band banding have been to select wafers of good material quality, deep implantation of N+ dopant up to the buffer or substrate at the source and drain regions, or implanting a p-type buried layer under the channel.
Each of these techniques has disadvantages such as lowering yield or generating additional deep traps.